All vertebrate species experience substantial changes in body size, musculature, neuronal and network properties during development. Despite these changes, motor behaviors must continue to be generated appropriately. In order to efficiently maintain movements throughout development, motor circuits in the spinal cord presumably stabilize excitatory drive to the motoneurons driving behavior. How motor circuits accomplish this task is unknown. However, a better understanding of this process could provide invaluable insight into developmental disorders that result from disruptions in motor network connectivity and excitability. Therefore, the goal of this proposal is to define the mechanisms by which spinal premotor excitatory drive is modified through development to generate consistent motor output. The zebrafish model system is ideally suited for the investigation of developing motor behaviors. Their transparency enables unprecedented in vivo access to developing motor circuitry, including longitudinal imaging of neurons and synaptic connections and recordings of neuronal activity and motor output. In zebrafish, as in other vertebrates, the major source of spinal premotor excitatory drive arises from glutamatergic V2a interneurons. Critically, at early, embryonic and later, larval stages the most dorsally located and earliest born V2a cells (dV2as) and 'primary' motoneurons (pMNs) are activated during the same types of strong movements despite dramatic changes in the size and electrical properties of spinal populations. These observations suggest that the role of the dV2as in driving pMNs during strong movements is maintained during development. This proposal will address how dV2as maintain connectivity and stabilize synaptic drive to pMNs through these developmental changes to support network function. In Aim 1, we will first consider how synaptic contacts are made and maintained by morphological assessment of axon and synapse distribution and stability. In Aim 2, we will examine the functional maturation of the dV2a-pMN connection by performing whole-cell patch clamp recordings at distinct stages of development. Our pilot data suggest that increases in pMN size (and thus decreases in input resistance) are accompanied by increases in excitatory drive. These experiments will help determine if this relationship can be explained by the strengthening of individual dV2a cell connections or the addition of new connections. Finally, in Aim 3 we will investigate the instructional role of postsynaptic excitability in setting presynapti strength by decreasing or increasing the excitability of pMNs through exogenous expression of ion channels. Together, the experiments outlined in this proposal will provide key insight into the morphological and functional mechanisms responsible for stabilizing function in an identifiable motor circuit during development.